Researchers had humans and dogs—both natural-born runners—jog a half hour on a treadmill. Then they sampled their blood for endocannabinoids, some of the compounds thought to trigger the runner's high. As expected, humans and dogs had much higher levels after the run. But when ferrets—a sedentary species—took the same 30-minute trot, they had no spike in those feel-good molecules.

The authors say that's because long-distance running could have helped our hunter-gatherer ancestors find more food—thus increasing their reproductive success. And they speculate that natural selection may have linked up a feel-good reward to that beneficial behavior. These days of course, this ancient trait won't help us find extra calories—but it may encourage us to run 'em off.